This invention relates to an electron gun assembly, and more particularly to improvements on an in-line type electron gun assembly formed of a plurality of electron guns emitting a plurality of electron beams.
An in-line type electron gun assembly used for a color picture tube is generally constructed as illustrated in FIG. 1. The electron gun assembly 11 comprises a first electrode 1, a second electrode 2, a third electrode 3, a fourth electrode 4 and cathodes 5R, 5G, 5B. The cathodes 5R, 5G, 5B emit a red electron beam 6R, a green electron beam 6G and a blue electron beam 6B respectively. These electron beams 6R, 6G, 6B pass through the holes of a shadow mask 7 and impinge on a phosphor screen 8.
With the electron gun assembly constructed as described above, the conventional process of converting three red, green and blue electron beams 6R, 6G and 6B at one point on the shadow mask 7 is to refract both side electron beams, that is, a red electron beam 6R and a blue electron beam 6B to the central green electron beam 6G at a convergence angle 9 at the fourth electrode 4. This arrangement is carried out by causing the axis 4C of both side holes of the fourth electrode 4 to be displaced from the axis of the central green electron beam 6G at a larger rate than that at which the axis 3C of both side holes of the third electrode 3 is set apart from the axis of the central green electron beam 6G. There will now described by reference to the red electron gun 5R of FIG. 2 the principle by which the above-mentioned arrangement can be attained. The red electron beam 6R does not pass exactly along the axis of the thick main electrostatic lens (formed of the third and fourth electrodes 3R, 4R overlapping each other), but along a line slightly displaced from said lens axis. Therefore, the red electron beam 6R is refracted downward at the fourth electrode 4. This downward refraction results from the optical principle illustrated in FIG. 5. A light beam 21 passes through a point slightly displaced from the center of, for example, a single convex glass lens 20 and consequently is refracted.
The convergence angle 9 which should be taken into account in designing a color picture tube is defined by causing the axes of both side holes of, for example, the third and fourth electrodes to be properly displaced from the axis of the central green electron beam 6G. In this case, the red electron beam 6R is sharply refracted by the main electrostatic lens formed of the third and fourth electrodes 3R, 4R. Therefore, the red electron beam 6R (also blue electron beam 6B) tends to be readily affected by the broadly defined spherical aberration, particularly coma abberration. As the result, a red electron beam image takes a tadpole trailing form 10R on the phosphor screen 8. A prior art attempt to reduce the coma aberration was to bulge the plane of the fourth electrode 4 parallel to the surface of the drawing as viewed in the vertical direction to relieve the sharp refraction of an electron beam, thereby enabling the image of said electron beam to approximate the true round form 10G of the central green electron beam 6G. However, the abovementioned proposed process which renders the image subject to strong distortion essentially presents difficulties in decreasing the effect of the coma aberration.